1. Field of the Invention
The present invention relates to an active matrix LED pixel driving circuit and particularly to an active matrix OLED/PLED pixel driving circuit.
2. Description of the Prior Art
Organic light emitting diodes (OLEDs) or polymer light emitting diodes (PLEDs) are more and more popularly used in flat displays due to their high speed performance, low power consumption and low cost. The OLED/PLED displays also have a wider angle of view than that of conventional liquid crystal displays using backlight systems since the OLEDs or PLEDs emit light themselves.
There are two categories of LED display, passive and active matrix. In the passive matrix display, each LED is provided with a driving current for only one scan period in one frame and is turned off until beginning of the scan period in the next frame. Each LED emits light strong enough in each short scan period to achieve a satisfied overall illumination level of the display. Thus, a large driving current is necessary. However, the large driving current shortens the lifetime of the LEDs as well as inducing a large power consumption.
On the contrary, the active matrix LED display does not have the previous drawbacks. It uses capacitors charged by the driving current during the scan period and keeping voltages thereon until the scan period of the next frame. These voltages allow currents driving LEDs to be turned on after the end of the scan period. Thus, the LEDs are turned on for a longer time period and the driving current can be lower than that of the passive matrix display.
The LEDs in the display can be driven by voltages or currents. FIG. 1 is a diagram showing one voltage-driven pixel circuit in an active matrix LED display. It comprises transistors 11 and 12, a capacitor 13, and an LED 14. The gate of the transistor 11 receives a scan signal SS through a scan line while its source receives a data signal DS through a data line. The data signal for this pixel is transmitted to the gate of the transistor 12 when the transistor 11 is turned on by the scan signal SS. If this pixel is lit in the current frame, the voltage level of the data signal DS turns on the transistor 12 to generate a driving current through the transistor 12 lighting the LED 14. In the meantime, the capacitor 13 is charged and keeps a voltage Vgs thereon. The voltage Vgs succeeds the data signal DS to keep the transistor 12 turned on when the scan signal SS turns off the transistor 11 to terminate transmission of the data signal DS at the end of the scan period. However, the pixel circuit in FIG. 1 suffers drift of the threshold voltage Vt which may result in drift of the driving current. The magnitude differences between the driving currents in the pixels lead to non-uniform illumination on the display panel.
FIG. 2 is a diagram showing one current-driven pixel circuit in an active matrix LED display. It comprises transistors 21, 22, 23 and 24, a capacitor 25, and an LED 26. The gate of the transistor 21 receives a scan signal SS through a scan line while its source receives a data signal DS through a data line. The gate of the transistor 22 also receives the scan signal SS. When the transistors 21 and 22 are turned on by the scan signal SS, the transistors 23 and 24 act as a current mirror so that the current through the transistor 23 is reproduced and flows through the transistor 24 to light the LED 26. In the meantime, the capacitor 25 is charged and keeps the voltage Vgs of the transistor 24 thereon. The voltage Vgs succeeds the data signal DS to keep the transistor 24 turned on when the scan signal SS turns off the transistors 21 and 22 to terminate transmission of the data signal DS at the end of the scan period.
FIG. 3A is a diagram showing another current-driven pixel circuit in an active matrix LED display. It comprises transistors 31, 32, 33 and 34, a capacitor 35, and an LED 36. The gate of the transistor 31 receives a scan signal SS through a scan line while its source receives a data signal DS through a data line. The gates of the transistors 32 and 33 also receive the scan signal SS. When the transistors 31 and 32 are turned on and the transistor 33 is turned off by the scan signal SS, the gate and drain of the transistor 34 are electrically connected, and the voltage Vgs is generated and has a magnitude corresponding to the current through the data line and the transistor 34. In the meantime, the capacitor 35 is charged and keeps the voltage Vgs thereon. The voltage Vgs succeeds the data signal DS to keep the current through the transistors 33 and 34 lighting the LED 36 when the scan signal SS turns off the transistors 31 and 32 and turns on the transistor 33 to terminate transmission of the data signal DS at the end of the scan period. FIG. 3B is a diagram showing a modified configuration of the circuit in FIG. 3A. They are similar in circuit operation. The PMOS transistor 34 is replaced by a NMOS transistor and the capacitor 35 exchanges with the transistor 32.
FIG. 4A is a diagram showing another current-driven pixel circuit in an active matrix LED display. It comprises transistors 41, 42, 43 and 44, a capacitor 45, and an LED 46. The gate of the transistor 41 receives a scan signal SS through a scan line while its source receives a data signal DS through a data line. The gate of the transistors 42 and 43 also receive the scan signal SS. When the transistors 41 and 42 are turned on and the transistor 43 is turned off by the scan signal SS, the gate and drain of the transistor 44 are electrically connected, and the voltage Vgs of the transistor 44 is generated and has a magnitude corresponding to the current through the data line, the transistors 41 and 44, and the OLED 46. The capacitor 45 is charged and keeps the voltage Vgs thereon. The voltage Vgs succeeds the data signal DS to light the OLED 46 by generating a current through the transistor 44 when the scan signal SS turns off the transistors 41 and 42 and turns on the transistor 43 to terminate transmission of the data signal DS at the end of the scan period. FIG. 4B is a diagram showing a modified configuration of the circuit in FIG. 4A. They are similar in circuit operation. The PMOS transistor 44 is replaced by a NMOS transistor and the capacitor 45 exchanges with the transistor 42.
FIG. 5 is a diagram showing an equivalent circuit of all the current-driven pixel circuits described previously. It comprises a transistor 51, a capacitor 52, a current switch 53 and an LED 54. The current switch 53 comprises three switches 531˜533, and a data line connected to a current source (not shown). The switches 531˜533 are controlled by the scan signal SS. The current source connected with the data line provides currents driving the LED 54.
At the beginning of the scan period, the switches 531 and 532 are closed and the switch 533 is opened. If this pixel is lit in the current frame, the current I of the data signal DS flows through the transistor 51 and charges the capacitor 52 to keep the voltage Vgs thereon. When the scan signal opens the switches 531 and 532 and closes the switch 533, the voltage Vgs succeeds the data signal DS to light the LED 54 by generating a current I′ through the transistor 51.
The current-driven pixel circuits described previously still suffer disadvantages resulting from channel length modulation although the drift of the threshold voltage has no significant impact on the illumination uniformity. As shown in FIG. 6, curves L21, L22 and L23 indicate the I-V characteristics of three transistors with different threshold voltages during the scan period (with the gate and drain connected). The line L1 indicates the I-V characteristics of the current source connected to the data line for transmission of the data signal DS. The magnitudes of the drain-to-source current and gate-to-source voltage of the transistors in a steady state during the scan period can be derived from the intersections a1, b1 and c1 of the curves L21, L22 and L23, and the line L1. As shown in FIG. 6, curves L31, L32 and L33 indicate the I-V characteristics of the three transistors beyond the scan period (with the gate and drain isolated from each other). The curve L4 indicates the I-V characteristics of the LED. The magnitudes of the drain-to-source current and gate-to-source voltage of the transistors in a steady state beyond the scan period can be derived from the intersections a2, b2 and c2 of the curves L31, L32 and L33, and the line L4. It is noted that the channel length modulation results in non-overlapping of the curves L31, L32 and L33 so that the magnitudes of the drain-to-source current of the three transistors are the same during the scan period but shifted to different values beyond the scan period. Thus, the illumination on the display panel is non-uniform due to the channel length modulation even if the threshold voltages of the transistors are the same.